In recent years, internal combustion engines, capable of operating in a lean burn mode with a fuel-lean mixture, have been developed to achieve further improved fuel economy. In this type of internal combustion engine, it is difficult to reduce NOx in exhaust gases during the lean burn operation, by using a conventional three-way catalyst having a function of reducing CO, HC, and NOx in exhaust gases when the engine is operating at around the stoichiometric air/fuel ratio.
In view of the above problem, a new type of catalyst (called adsorption type lean NOx catalyst or trap-type lean NOx catalyst) has been developed which has a function of adsorbing NOx in exhaust gas under an oxygen excessive atmosphere (or oxidizing atmosphere), and releasing the adsorbed NOx when the oxygen concentration of the exhaust gas is reduced.
More specifically, the lean NOx serves to oxidize NOx in the exhaust gas to produce nitrates when it is surrounded by an atmosphere having a high oxygen concentration, thereby to adsorb NOx, and also serves to produce carbonates by causing reaction between the nitrates, adsorbed on the lean NOx catalyst, and CO in the exhaust gas when it is surrounded by a reducing atmosphere having a reduced oxygen concentration, thereby to release NOx. The NOx thus released from the lean NOx catalyst is then converted into a harmless substance due to the three-way function of the lean NOx catalyst, or by means of a three-way catalyst located downstream of the lean NOx catalyst.
While NOx in the exhaust gas can be surely reduced by the lean NOx catalyst during the lean burn operation, it is still difficult to sufficiently reduce HC in the exhaust gas when the engine starts operating in a cold state, for example, only by using the lean NOx catalyst as described above.
In order to reduce HC in the exhaust gas to a sufficiently low level upon start of the engine in a cold state, it has been proposed to provide a light-off catalyst (L/O catalyst, FCC: Front Catalytic Converter) immediately downstream of the engine and upstream of the conventional catalyst.
Examples of the light-off catalyst are disclosed in, for example, Japanese laid-open Patent Publication No. 8-294618 (first publication), and Japanese laid-open Patent Publication No. 5-187230 (second publication).
As disclosed in the above-identified first and second publications, the light-off catalyst consists of a three-way catalyst (TWC), and the three-way catalyst or oxidizing catalyst used as the light-off catalyst contains, for example, ceria (CeO2) as an additive having a function of storing O2.
While the engine operates in a stoichiometric feedback mode or a lean burn mode during normal running of the vehicle, it sometimes operates in a rich burn mode with a fuel-rich mixture when the vehicle is in a transient state, for example, when the vehicle is accelerating. In this case, since the amount of O2 contained in the exhaust gas is not sufficient to oxidize HC and CO, O2 stored in ceria (CeO2) of the light-off catalyst is utilized for oxidizing HC and CO, thereby to reduce HC to a sufficiently low level even during the transient rich burn operation of the engine.
The use of both the lean NOx catalyst and the light-off catalyst as disclosed in the above first and second publications, however, suffers from a problem as follows: since the light-off catalyst has an O2 storage function, CO that is needed for releasing adsorbed NOx from the lean NOx catalyst in a reducing atmosphere is undesirably oxidized by the light-off catalyst, and a sufficient amount of CO cannot be supplied to the lean NOx catalyst. Thus, the NOx adsorbed on the lean NOx catalyst may not be sufficiently released from the catalyst because of the oxidation of CO by the light-off catalyst.
In order to recover or resume the NOx conversion efficiency of the lean NOx catalyst to a nominal level, recovery control is performed to produce a rich atmosphere (of a small air-fuel ratio) having a reduced oxygen concentration around the lean NOx catalyst, thereby to release the adsorbed NOx from the lean NOx catalyst. As an example of the recovery control, an additional fuel is injected to the combustion chamber of the engine. Since CO supplied by the recovery control is oxidized and consumed by O2 stored in the additive (for example, ceria CeO2) of the light-off catalyst, NOx adsorbed on the lean NOx catalyst cannot be surely released from the catalyst, and the NOx conversion efficiency of the lean NOx catalyst cannot be resumed to the nominal level.
It may be considered to control the air-fuel ratio to be even richer in order to resume the NOx conversion efficiency of the lean NOx catalyst to the desired level, but this method results in deterioration of the fuel efficiency or fuel economy, and is therefore undesirable.
In the meantime, a sulfur component (S component) is contained in the fuel and lubricating oil, and such a sulfur component is also contained in exhaust gas. The lean NOx catalyst, therefore, serves to adsorb the sulfur component as well as NOx in a high oxygen concentration atmosphere. Namely, the sulfur component contained in the fuel or lubricating oil is burned in the combustion chamber, and then oxidized on the surface of the lean NOx catalyst, to provide SO3. A part of the SO3 then reacts with an adsorbent for adsorbing NOx on the lean NOx catalyst, to produce sulfates that are adsorbed on the lean NOx catalyst.
While the nitrates and sulfates as described above are adsorbed on the lean NOx catalyst, the amount of sulfates remaining on the lean NOx catalyst increases with time, since the sulfates have higher stability in the form of salts than nitrates, and only a part of the sulfates dissolves in an atmosphere having a reduced oxygen concentration. With an increase in the remaining amount of sulfates, the ability of the lean NOx catalyst to adsorb NOx deteriorates with time, and the purifying efficiency (NOx conversion efficiency) of the lean NOx catalyst is decreased. This is called S poisoning.
When the lean NOx catalyst suffers from the S poisoning as described above, it becomes necessary to release the sulfur component (SOx) from the lean NOx catalyst. Where both of the lean NOx catalyst and the light-off catalyst, as disclosed in the above first and second publications, are used, however, SOx adsorbed on the lean NOx catalyst cannot be sufficiently released since the light-off catalyst has a high O2 storage ability.
To regenerate the lean NOx catalyst by producing an atmosphere having a reduced oxygen concentration around the lean NOx catalyst, and releasing SOx adsorbed on the lean NOx catalyst in the reducing atmosphere, regeneration control is performed by, for example, controlling the air-fuel ratio to be richer, to reduce the oxygen concentration of the exhaust gas. Since CO supplied under the regeneration control for releasing SOx reacts with O2 stored in an additive (for example, ceria CeO2) of the light-off catalyst, to be oxidized and consumed by the O2, the SOx adsorbed on the lean NOx catalyst cannot be released from the catalyst, and the lean NOx catalyst cannot be regenerated as desired.
Also, SO2 emitted from the engine reacts with O2 stored in the additive of the light-off catalyst, to produce SO3 (2SO2+O2═2SO3), and therefore the sulfur component is likely to be adsorbed on the lean NOx catalyst located downstream of the light-off catalyst. Thus, the lean NOx catalyst is more likely to suffer from S poisoning due to the oxidizing function of the light-off catalyst.